1. Field of the Invention
The present invention relates to a clutch piston for a lock-up clutch in a torque converter, and particularly, to an improvement in a clutch piston having a friction lining, which piston divides a lock-up clutch chamber defined between a back of a turbine impeller and a side cover connected to a pump impeller to cover the back of the turbine impeller, into an inner chamber section on the side of the turbine impeller and an outer chamber section on the side of the side cover; the piston is axially movably connected to the turbine impeller such that it is moved under the influence of a difference in pressure between the inner chamber section and the outer chamber section between a clutch-ON position where it is in pressure contact with an inner wall of the side cover, and a clutch-OFF position where it is spaced apart from the inner wall; and the piston is bonded at its end surface opposed to the inner wall with an annular friction lining having an entirely flat friction surface. The present invention also relates to a process for producing such a clutch piston having a friction lining.
2. Description of the Related Art
Such lock-up clutch in a torque converter is constructed such that it is controlled into a turned-ON state in a coupling area to connect the pump impeller and the turbine impeller directly to each other, thereby eliminating a slip between both the impellers to provide an enhancement in transmitting efficiency. Such a lock-up clutch is known, for example, as disclosed in Japanese Patent Application Laid-open No. 5-71610.
The turned-ON state of the lock-up clutch is achieved by bringing the inner chamber section of the lock-up clutch chamber into communication with the working oil supply side and bringing the outer chamber section into communication with a low-pressure side, thereby urging the clutch piston toward the inner wall of the side cover under the influence of the difference in pressure to bring the friction lining into friction engagement with the inner wall.
In this case, it is possible to avoid leakage of hydraulic pressure in the inner chamber section through between the friction lining and the side cover toward the outer chamber section thereby to ensure a good clutch-ON state, because the annular friction lining of the clutch piston has an entire friction surface formed flat, unlike a friction clutch plate of a wet multi-plate clutch mounted in a usual transmitting system, which plate has an oil groove formed in its friction surface.
In the prior art, the annular friction lining having its entire friction surface formed flat is formed of a single seamless member made in an annular shape by punching of a lining blank sheet. However, if such a friction lining is employed, scraps are produced in a large amount from the lining blank sheet, resulting in a very poor yield of the blank sheet. This is a large obstacle to the reduction in cost for clutch pistons having friction linings.
Accordingly, it is an object of the present invention to provide a clutch piston having a friction lining for a lock-up clutch, which is produced at a low cost and operated reliably, wherein the friction lining is of an annular shape and formed of lining segments made by punching a lining blank sheet at a good yield. It is another object of the present invention to provide a process for producing such a clutch piston having a friction lining.
To achieve the above object, according to a first aspect and feature of the present invention, there is provided a clutch piston having a friction lining for a lock-up clutch, dividing a lock-up clutch chamber defined between a back of a turbine impeller and a side cover connected to a pump impeller to cover the back of the turbine impeller, into an inner chamber section on the side of the turbine impeller and an outer chamber section on the side of the side cover; which is axially movably connected to the turbine impeller such that the clutch piston is moved under the influence of a difference in pressure between the inner chamber section and the outer chamber section between a clutch-ON position where the clutch piston is in pressure contact with an inner wall of the side cover, and a clutch-OFF position where the clutch piston is spaced apart from the inner wall; the clutch piston being bonded at an end surface thereof opposed to the inner wall with an annular friction lining having an entirely flat friction surface, wherein the friction lining is formed of a plurality of fan-shaped lining segments arranged annularly without gaps therebetween.
With the above arrangement, the annular friction lining is formed of the fan-shaped lining segments and hence, the friction lining can be produced at a good yield from a lining blank sheet by regularly punching the lining blank sheet into the lining segments, which contributes largely to a reduction in cost. Moreover, there is no gap between adjacent lining segments and hence, in a turned-ON state of the clutch, it is possible to avoid leakage of hydraulic pressure from the inner chamber section of the lock-up clutch chamber through between the lining segments toward the outer chamber section, thereby maintaining a large difference in pressure between both the chamber sections and ensuring a good clutch-ON state.
According to a second aspect and feature of the present invention, in addition to the first feature, each of the lining segments is bonded to the clutch piston with an adhesive, and end surfaces of adjacent lining segments are bonded to each other with the adhesive.
With the above arrangement, each of the lining segments can be bonded reliably to the clutch piston by the adhesive, and moreover the gap can be eliminated reliably from areas between the lining segments, thereby enhancing the performance of inhibiting the leakage of hydraulic pressure.
According to a third aspect and feature of the present invention, there is provided a process for producing a clutch piston having a friction lining for a lock-up clutch, which is of the above-described type, comprising the steps of: applying an adhesive to at least one of opposed surfaces of the clutch piston and each of the lining segments; placing the plurality of lining segments arranged annularly onto the clutch piston with the adhesive interposed therebetween; and pressing the lining segments against the clutch piston by a pressing jig having a flat pressing surface, thereby bonding each of the lining segments to the clutch piston and at the same time, bringing end surfaces of adjacent lining segments into close contact with each other by the compression and deformation of all the lining segments, and shaping that the surfaces of all the lining segments to be of one plane.
With the above feature, it is possible to provide, at a low cost, a clutch piston having a friction lining, which exhibits good performance of inhibiting the leakage of hydraulic pressure in the clutch-ON state.
According to a fourth aspect and feature of the present invention, in addition to the third feature, further comprising the step of causing the adhesive present between the clutch piston and the lining segments to be diffused to areas between the end surfaces of adjacent lining segments, at the time of pressing the lining segments against the clutch piston by the pressing jig.
With the above feature, each of the lining segments can be bonded reliably to the clutch piston by the adhesive, and the gap can be eliminated reliably from areas between the lining segments.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.